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Lecture 27 - Test 2

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This portion of the test is worth 30 total points. You may reference any files within your directory, as well as the Internet. As usual, you are not allowed to directly copy any code from the Internet or another individual without citation. You should also follow all coding standards discussed thus far.

Create a directory called test2 under cs120. Make sure all files you work on today are stored in this directory.

cd cs120
mkdir test2
cd test2

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Question 10

Create a file called question_10.py. Put your answers to the following questions there.

(6 points each, 30 points total)

• Write a function called compute_discount. This function takes an amount of items purchased, and returns a floating point value representing their discount percentage. You can compute the percentages using the following table:

  Number of Items	Discount
  5 or fewer	0%
  6 - 10	5%
  11 - 20	8%
  21 - 25	12%
  25 - 30	17%
  30+	20%

    >>> print(compute_discount(6))
    0.05
    >>>

• Write a function is_prime, which takes a single parameter: an integer greater than 1. This function should return True if and only if the parameter is prime. It should return False in all other instances. Recall that a number n is prime if is not divisible by any number in the range [2, n).

  Challenge

  If you noticed, I only said "…if is not divisible by any number…" That's because the condition is sufficient, but not exactly necessary. Modify your code so that it only checks the necessary cases.

  (2 bonus points)

    >>> print(is_prime(17))
    True
    >>> print(is_prime(21))
    False
    >>>

• Write a function test_arithmetic. This function should not take any parameters. Your function should generate two random (using the random module) positive integers in the range [0, 100], and ask the user to type in the sum of the two numbers generated. This process should continue until the user gets a question wrong, or the user enters a -1. Your function should print a statement of the form:

    You answered 4 questions correctly out of 5.

  You should not count the question that the user gives up against them.

    You answered 0 questions correctly out of 0.

• Write a function hash(string) that takes a string parameter, and returns the normalized sum of the ASCII values of the characters in the parameter string. In this case, consider a sum normalized if the sum is restricted to the range [0, 2⁸), using the remainder operator.

    >>> print(hash("This is a test case"))
    177
    >>>

• Using none of the built in String methods, write a function called strip_chars(string, chars), which takes as a parameter two strings, and returns a copy of the input string with all of the characters contained in the string chars removed.

    >>> print(strip_chars("This is a test case", "This "))
    atetcae
    >>>

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Bonus

Vigenère's Cipher

Vigenère's Cipher is very easy to understand, but is incredibly difficult to break. It took three centuries before anyone was able to break it. This earned its title: le chiffre indéchiffrable (the indecipherable cipher).

Vigenère's cipher is really just an extension of Caesar's Cipher. The main difference is the key that is used. In Caesar, the key is an integer representing the shift. Vigenère used letters to specify the shift. In fact, Vigenère's key is an entire word. The index of the letter in the Alphabet represents the shift performed using Caesar's cipher.

The added security comes from the fact that now each letter of the plain text is getting encrypted using different shift amounts, being specified by the key word. Consider the key "dawn". Using this key, you would encrypt the first letter of the plain text, performing Caesar with the key 3. Then, you would encrypt the second letter with the key 0. The third letter gets encrypted with the key 22. The fourth letter gets encrypted with the key 13. Then the cycle repeats: letter five uses the key 3, letter six uses the key 0, letter seven uses the key 22, and letter eight uses the key 13, and so on.

Create a file called bonus_question.py. In this file, write a function called encode_vigenere, which takes two parameters: plaintext and key, both strings. Your function should return a string, the Vigenère encoding of the plain text using the specified key.

(10 points)

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Submission

When you have finished, create a tar file of your test2 directory. To create a tar file, execute the following commands:

cd ~/cs120
tar czvf test2.tgz test2/

To submit your activity, go to cseval.roanoke.edu. You should see an available assignment called Test 2.

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Last modified: Mon Nov 4 09:45:47 EST 2013